Hey!
So let me give you an overview over those you picked:
Lille: Lille is north of Paris and very close to Belgium. It's a beautiful city but maybe not very typically French, a little Belgish already. As I said, Belgish Border is about 10 min away by car. The city centre is really beautiful. And concerning the foreign countries to visit: You know that the Eurostar is a very fast trian that goes under the Channel Sea from Paris to London in 2 hours right? And Lille is exactly half way between Paris and London. So both cities are about 1 hour away by train. So the position is really great. And also Brussels, the Capital City of European Union is about 1 hour away. I don't know when you go, but during summer the beach is about 1 h away from Lille, but it could be quite cold up there.
Toulouse: Toulouse is a very beautiful city in Southern France. Usually it's nice and warm, the city centre is very beautiful and the houses are all a shade of light pink so it's called the "Pink City". :-) You are right, Spain is not very far away, but there are the Pyrrenian Mountains in between so it might be quite long to reach it but I would say to Spanish Border about 2 h by car from Toulouse. But there is one further advantage: The city is about halfway between the Mediterranean Sea and Atlantic Ocean, so you can reach both seas in about 1,5 h. And if you are interested in airplanes and technique, you know in Toulouse they build many Aircrafts and it is very famous for that Also there are nice museums concerning that topic.
Rouen: Rouen is another very beautiful city in Normandy Region. It is situated about 1 h by train northwest of Paris and also around 1 h from the coast. So a beach wouldn't be that far and neither would Paris be. In Rouen you will find a great Cathedral, very beautiful old city centre and close to Rouen was the house where the painter Claude Monet lived.
Lyon: This is also a great city, it's the second largest city of France and it's situated in the South-East of the country. Lyon is very famous for its great historic city centre. Paris is reachable in about 3 hours by train I think, but you should check it once again, I'm not absolutely sure. Concerning neighbor countries: Lyon is very close to Genève in France (about 1 h by car/train) which is a very beautiful city on Lac Leman Lake. And if you want to go skiing in winter, the ski areas of French Alps are very close to Lyon. Since the new train connection for some years now you can reach the Southern French CIty of Marseille in a very short time by trian (high-speed train TGV) and get in touch with the sea. And around Lyon you have the nice cities of Annecy and Chambery.
Nantes: Nantes is a city in the West of France. It is not so far from Paris (maybe around 2 h by train) and it is very beautiful too. The sea is very close and also the beautiful Loire Castles are not far away. Brittany (in French Bretagne) is also just around 1 h away and I think Bretagne is among the most beautiful areas of France. Along the Coastline of Atlantic Ocean there are many beaches and it's famous for windsurfing.
Paris: Well, Paris is probably the most beatufiul French City. It's not just my home city :-)) but very beautiful and you will never be bored here. It's just such a great place to be... :-) Around Paris you can also find many beautiful sights like Versailles Castle of the Sun King Louis XV or the famous Disneyland. The beach is about 1-2 h away and concerning your bordering country: Paris is not really close to any border, but the TGV high-speed train has a great connection to London (2 h) via Lille. The trian goes through a tunnel under the sea. And I know people that go there just for 1 day from time to time and in the evening they come back. But be sure to make a reservation in advance.
And if I can make you one more recommendation: Nice is a very beautiful city. It is situated in the South-East of the country on the Mediterranean Sea. Around Nice is the famous French Riviera (Côte d'Azur) so you will find great beaches, a very warm and Mediterranean Climate and great places to visit: Cannes, Monaco, Saint-Tropez, Antibes,... Italy is just about half an hour away by train. And if you want to go skiing it's also just around 1 h by car. And in Nice you can take advantage out of a nice effect: In winter it's still very warm in Nice, but in the mountains just behind Nice the climate changes and it's very cold. So you could go skiing in say Valberg and then go to Nice in less than 1 hour and enjoy the evening in just a T-Shirt on the Beach. If you're lucky you could also swim, because the sea never gets really cold here. So it's just a great place to be.
I hope I could give you a nice overview of the cities you chose. If I had to choose I would go to Paris, Rennes or Nantes. But pleae choose yourself, because they are all great cities.
Hope I helped and pleae enjoy France,
Milan